This invention relates to a catalytic cracking process and is particularly concerned with the cracking of feedstocks containing substantial quantities of nitrogen-containing compounds.
Fluidized catalytic cracking (FCC) units are used in the petroleum industry to convert high boiling hydrocarbon feedstocks to more valuable hydrocarbon products, such as gasoline, having a lower average molecular weight and a lower average boiling point than the feedstocks from which they were derived. The conversion is normally accomplished by contacting the hydrocarbon feedstock with a moving bed of catalyst particles at temperatures ranging between about 800.degree. F. and about 1100.degree. F. The most typical hydrocarbon feedstock treated in FCC units comprises a heavy gas oil, but on occasions such feedstocks as light gas oils or atmospheric gas oils, naphthas, reduced crudes and even whole crudes are subjected to catalytic cracking to yield low boiling hydrocarbon products.
Catalytic cracking in FCC units is generally accomplished by a cyclic process involving separate zones for catalytic reaction, steam stripping, and catalyst regeneration. The hydrocarbon feedstock is blended with an appropriate amount of catalyst particles to form a mixture that is then passed through a catalytic reactor, normally referred to as a riser, wherein the mixture is subjected to a temperature between about 800.degree. F. and about 1100.degree. F. in order to convert the feedstock into gaseous, lower boiling hydrocarbons. After these gaseous, lower boiling hydrocarbons are separated from the catalyst in a suitable separator, such as a cyclone separator, the catalyst, now deactivated by coke deposited upon its surfaces, is passed to a stripper. Here the deactivated catalyst is contacted with steam to remove entrained hydrocarbons that are then combined with vapors exiting the cyclone separator to form a mixture that is subsequently passed downstream to other facilities for further treatment. The coke-containing catalyst particles recovered from the stripper are introduced into a regenerator, normally a fluidized bed regenerator, where the catalyst is reactivated by combusting the coke in the presence of an oxygen-containing gas, such as air, at a temperature which normally ranges between about 1000.degree. F. and about 1500.degree. F. The cyclic process is then completed by blending the reactivated catalyst particles with the feedstock entering the riser or reaction zone of the FCC unit.
It is well known that catalytic cracking feedstocks which contain high levels of nitrogen have a deleterious effect on cracking catalysts. The nitrogen is typically present in the form of basic or neutral organic compounds, primarily aromatic compounds containing nitrogen heteroatoms, such as pyridines, indoles and quinolines, which are strongly sorbed on the acidic sites of the cracking catalyst. The nitrogen compounds react or otherwise interact with the acidic sites so as to decrease the activity of the catalyst. This deactivation results in decreased conversions and gasoline production. Levels of nitrogen in the feedstock as small as 0.01 weight percent, calculated as the element, can result in some decrease in activity of the catalyst; however, significant deactivation is not normally encountered unless the concentration of nitrogen in the feedstock increases to about 0.08 weight percent or above. Nitrogen poisoning of cracking catalysts is quite severe when the feedstock is a synthetic oil derived from carbonaceous solids such as oil shale, coal, tar sands and the like. Such synthetic oils tend to have relatively high concentrations of nitrogen, sometimes ranging as high as 5.0 weight percent, calculated as the element.
In order to avoid substantial deactivation of cracking catalysts by nitrogen compounds in feedstocks containing high levels of nitrogen, it has been standard practice to treat such feedstocks to reduce the concentration of basic nitrogen compounds prior to subjecting the feedstocks to catalytic cracking. Techniques employed in the past for reducing the concentration of basic nitrogen compounds include (1) adsorbing the compounds on solid material such as silica, alumina or various grades of clay, (2) treating the feedstock with mineral acids, such as hydrochloric and sulfuric acid, to form soluble salts of the basic nitrogen compounds, and (3) treating the feedstock in the presence of added hydrogen with a hydrogenation catalyst.
Of the above-discussed techniques, the latter one, hydroprocessing, is the one most frequently used. However, in order to remove substantial quantities of residual nitrogen, hydrogenation pressures up to 5000 p.s.i.g. are typically required. Installation of equipment to carry out such a high pressure process requires a substantial capital investment. The use of solid materials to adsorb the nitrogen compounds from the feed also has disadvantages in that this method requires the installation of additional equipment and includes expensive solids handling techniques. Although treating the feedstock with mineral acids to form soluble salts of the basic nitrogen compounds in the feedstock may be effective in reducing nitrogen poisoning of the cracking catalyst, the use of mineral acids results in other serious problems. For example, if sulfuric acid or nitric acid is used, the resultant salts will decompose in the riser of the FCC unit to produce sulfur constituents and nitrogen constituents, which can react with steam in the stripper to form strong acids that can cause severe corrosion of the FCC unit internals and downstream equipment. Also, if these sulfur and nitrogen constituents deposit in the coke that forms on the catalyst in the riser, they will be converted to sulfur oxides and nitrogen oxides, which are undesirable air pollutants, when the coke-containing catalyst passes into the regenerator of the FCC unit.
Accordingly, it is one of the objects of the present invention to provide a fluid catalytic cracking process for treating feedstocks that contain relatively high concentrations of nitrogen constituents while maintaining the activity of the catalyst at a reasonable level. It is another object of the invention to provide such a process that will not result in increased emissions of pollutants from the regenerator. It is a further object of the invention to provide such a process which will not result in corrosion of the FCC unit internals. These and other objects of the invention will become more apparent in view of the following description of the invention.